The invention concerns the cutting machining of rotating, in particular concentric rotationally symmetrical surfaces of a metal workpiece, in particular of steel or cast iron, even in the hardened state, by means of a geometrically defined cutting edge or cutting edges.
The rotary machining of rotationally symmetrical parts both in the soft and also in the hardened state with a geometrically defined cutting edge is state of the art. The processes involving grinding, finishing, honing and similar processes are in part substituted in that respect.
Cutting materials which have a sufficient service life, even for machining hardened workpieces, are available in the meantime in various different forms.
Machining after the hardening operation is generally necessary, even if the preliminary machining operation is effected with a high degree of precision, as, after the hardening process, in part considerable degrees of distortion due to hardening generally occur. A process which restores the dimensional accuracy of the workpiece again is therefore essential.
Longitudinal turning always produces a surface which suffers from a spiral rifling twist thereon, more particularly, irrespective of which material is machined and whether it is hardened or unhardened.
That surface has regular structures (grooves or burr-shaped raised portions) which correspond to a screwthread structure (twist) and which, by virtue of the feed movement of the tool along the rotating workpiece, produce a pitch.
That applies equally to workpiece surfaces which are cylindrical, conical or of some other shape. Therefore the surface to be produced on the workpiece involves for example shapes of a screwthread or portions thereof.
The formula relationship which describes the screwthread depth or roughness of the thread-like structure is described in FIG. 3.
As the cutting machining data, particularly when dealing with hardened workpieces, for high levels of surface quality, are generally very small, this frequently involves a relatively slow machining advance or a low level of machining output.
While the relatively low level of machining output involves a marked economic disadvantage, the twist-bearing surfaces give rise to problems in relation to seals which bear thereagainst, particularly if they are moving relative to the surface, for example a rotating shaft within a stationary seal. Seals in that sense can be for example the known radial shaft sealing rings.
A surface which involves a twist configuration thereon in that fashion, by means of the thread-shaped grooves or burrs, conveys coolant, lubricant etc along the surface and past the seal bearing thereagainst, from one side to the other in the axial direction, so that the sealing action of the seal is considerably reduced. Particularly in the case of machines which for example for reasons of hygiene or also for reasons of environmental protection, must run without leakage, this represents a problem which is to be taken seriously.
In addition, the sealing elements which bear against the surface with the twist configuration thereon suffer a great deal of damage with time at the contact lines or surfaces therebetween, due to those thread-shaped grooves or raised portions, or they are at least subjected to a severe abrasive effect. Due to that wear or damage, the sealing action is also often seriously reduced or eliminated after a short period of time.
If consideration is given to the available cutting machining processes on a rotating workpiece, having regard to those two problematical aspects, the picture which results is as follows:
If, when turning rotationally symmetrical surfaces, the attempt is made to avoid the thread-shaped configuration being formed thereon by the tool being moved relative to the workpiece only radially (plunge-cut turning), then a twist-free surface is produced, because of the absence of any axial movement. If however the plunge-cut cutting edge is as wide in the axial direction as the rotationally symmetrical surface which is to be produced overall, very high cutting forces occur, particularly when machining hardened surfaces, and there is a high tendency to chatter because of dynamic instability. Such dynamic instabilities or chatter result almost all of a sudden in such severe surface irregularities that here the surface is also too irregular to afford satisfactory sealing integrity.
If the plunge-cut cutter is additionally moved lengthwise, that is to say in the axial direction, when dealing with relatively wide surfaces to be produced, then it will be appreciated that a surface with a twist configuration thereon is again produced.
It is therefore state of the art and necessary that the twist produced, in particular the thread-shaped turning grooves or tool marks have to be sufficiently reduced or even entirely eliminated with subsequent expensive additional procedures, in order thereby to ensure a satisfactory sealing action.
One possible way of avoiding the twist configuration on the surface (turning grooves or tool marks) could be the turning broaching process, wherein the broaching tool is moved in the tangential direction past the rotating workpiece. If the individual cutting edges of the turning broaching tool are oriented parallel to the axis of rotation of the tool, this procedure again involves the problems of the high forces acting on the cutting edge, and thus the trend towards dynamic instability and the tendency to chatter.
If the cutting edge is positioned inclinedly in the turning broaching operation, which results in a reduction in the pressure on the cutting edge however, that gives rise to a thread-shaped residual structure as the points of engagement of the cutting edge on the workpiece are at different spacings (radii) from the axis of rotation of the workpiece.
In a turning broaching operation by means of a disc-shaped tool with the cutting edges on the periphery of the tool, inclined positioning of the cutting edge which in itself is straight also gives rise to the problem that in that case the surface produced on the workpiece is a spherically convex surface instead of being an exactly cylindrical surface.
In addition in many cases grinding of the surfaces is applied as an additional procedure. That means that the workpiece generally has to be transposed to another type of machine. The item costs of the workpiece are therefore considerably increased by virtue of the increase in the length of the process chain, that is to say by the use of a further machine, and the economic result is thus significantly worsened. In addition, in regard to final machining of the workpieces, if possible grinding should be avoided as this generally takes place in the form of a wet process and thus further problems in terms of environment and disposal occur in regard to the grinding slurry and in accordance with the present state place a further burden on the economic result.
Added to that is the fact that even in the grinding operation twist structures are produced, which are firstly produced on the grinding disc by the dressing operation and which finally are reproduced on the workpiece. A finishing procedure in which a grinding band or a grinding element is applied to the workpiece also involves the formation of surface structures involving a twist configuration, because of the additional oscillation or longitudinal movement of the finishing tool relative to the workpiece.